Music industry uses vulnerability in Kazaa Hash Calculations

Found this on the k-lite forum. It explains the floud of damaged mp3s. Pure horror. We can’t trust the verifieds anymore.

Author: Hasnain
Date: 08-26-03 16:13

Foreword:

I am listing this topic here, because more and more people are beginning to download fake or corrupted files from the FastTrack network, despite using a verified hash. Some people have asked me how this is possible, the main reason being that Kazaa does not use each and every byte of a file to determine its hash.

Vulnerability:

I had noticed this vulnerability when saw the source code of sig2dat. Essentially, Kazaa calculates the hash of a file in the following way:

1. It reads the filesize and if the filesize is less than 300Kb, it hashes the whole file. If the filesize is greater than 300Kb it reads the first 300Kb from the file. The hash method is md5 one way hash.
2. After hashing the first 300Kb, it now calculates the offset of the next block as the offset of the previous block t shifted by two i.e.
New Offset = Old Offset

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